Hybrid polarizer

ABSTRACT

A hybrid polarizer includes an absorbing element having a first major surface and a second major surface. The hybrid polarizer also includes a first birefringent reflective polarizer disposed on the first major surface of the absorbing element, the first birefringent reflective polarizer having a first pass axis and a first block axis. The hybrid polarizer further includes a second birefringent reflective polarizer disposed on the second major surface of the absorbing element, the second reflective polarizer having a second pass axis and a second block axis.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/916,838, filed on Nov. 1, 2010, published as U.S. Patent ApplicationPublication No. 2011/0043732, now allowed, which is a continuation ofU.S. application Ser. No. 11/614,494, filed on Dec. 21, 2006, issued asU.S. Pat. No. 7,826,009, and incorporated herein by reference.

FIELD OF THE INVENTION

The present disclosure is directed to polarizers, and, for example,hybrid polarizers and display devices using hybrid polarizers.

BACKGROUND

Display devices, such as liquid crystal display (LCD) devices, are usedin a variety of applications including, for example, televisions,hand-held devices, digital still cameras, video cameras, and computermonitors. Because an LCD panel is not self-illuminating, some displayapplications may require a backlighting assembly or a “backlight.” Abacklight typically couples light from one or more sources (e.g., a coldcathode fluorescent tube (CCFT) or light emitting diodes (LEDs)) to theLCD panel.

Common display devices usually include polarizers. The most commonlyused type of a polarizer is a dichroic polarizer. Dichroic polarizersare made, for example, by incorporating a dye into a polymer sheet thatis then stretched in one direction. Dichroic polarizers can also be madeby uniaxially stretching a semicrystalline polymer such as polyvinylalcohol, then staining the polymer with an iodine complex or dichroicdye, or by coating a polymer with an oriented dichroic dye. Manycommercial polarizers typically use polyvinyl alcohol as the polymermatrix for the dye. Dichroic polarizers normally have a large amount ofabsorption of light.

Another common type of a polarizer used in displays is a reflectivepolarizer. Reflective polarizers tend to be more efficient intransmitting light of the high transmission polarization. This is due tothe use of a non-absorbing dielectric stack for polarizing light. Thesetypes of polarizers sometimes have defects, such as leakage of lightthrough localized areas of the sheet and incomplete reflectivity of thehigh extinction polarization over the wavelength region of interest.

In some displays applications, reflective and dichroic polarizers havebeen combined, as described, for example, in Ouderkirk et. al. U.S. Pat.No. 6,096,375 and Weber et. al. in U.S. Pat. No. 6,697,195, herebyincorporated by reference herein. The combination of the two polarizersprovides a high reflectivity of one polarization and high transmissionfor the perpendicular polarization for light incident on the reflectivepolarizer side of the combined polarizer, and high absorption andtransmission for light of orthogonal polarizations incident on thedichroic polarizer side.

SUMMARY

In one exemplary implementation of the present disclosure, a hybridpolarizer includes an absorbing element having a first major surface anda second major surface. The hybrid polarizer also includes a firstnearly uniaxial birefringent reflective polarizer disposed on the firstmajor surface of the absorbing element, the first nearly uniaxialbirefringent reflective polarizer having a first pass axis and a firstblock axis. The hybrid polarizer further includes a second birefringentreflective polarizer disposed on the second major surface of theabsorbing element, the second reflective polarizer having a second passaxis and a second block axis.

In another exemplary implementation, a hybrid polarizer includes anabsorbing polarizer having a pass axis and a block axis, a first majorsurface and a second major surface. The hybrid polarizer also includes afirst nearly uniaxial birefringent reflective polarizer disposed on thefirst major surface of the absorbing polarizer, the first nearlyuniaxial birefringent reflective polarizer having a first pass axis anda first block axis. The hybrid polarizer further includes a secondbirefringent reflective polarizer disposed on the second major surfaceof the absorbing polarizer, the second reflective polarizer having asecond pass axis and a second block axis.

In yet another exemplary implementation of the present disclosure, adisplay device includes a display panel and a hybrid polarizer. Thehybrid polarizer includes an absorbing element having a first majorsurface and a second major surface. The hybrid polarizer also includes afirst nearly uniaxial birefringent reflective polarizer disposed on thefirst major surface of the absorbing element, the first nearly uniaxialbirefringent reflective polarizer having a first pass axis and a firstblock axis. The hybrid polarizer further includes a second birefringentreflective polarizer disposed on the second major surface of theabsorbing element, the second reflective polarizer having a second passaxis and a second block axis. The first and second pass axes of thefirst and second reflective polarizers are substantially aligned.

In yet another exemplary implementation of the present disclosure, adisplay device includes a display panel and a hybrid polarizer. Thehybrid polarizer includes an absorbing element having a first majorsurface and a second major surface. The hybrid polarizer also includes afirst birefringent reflective polarizer disposed on the first majorsurface of the absorbing element, the first nearly uniaxial birefringentreflective polarizer having a first pass axis and a first block axis.The hybrid polarizer further includes a second birefringent reflectivepolarizer disposed on the second major surface of the absorbing element,the second reflective polarizer having a second pass axis and a secondblock axis. The first and second pass axes of the first and secondreflective polarizers are substantially aligned. The second reflectivepolarizer is disposed closer to the display panel than the firstreflective polarizer and the second reflective polarizer comprises aplurality of layers characterized by a varying optical thickness and amajority of the layers having a smaller optical thickness are disposedcloser to the display panel than the layers having a larger opticalthickness.

These and other aspects of the polarizers and display devices of thesubject invention will become more readily apparent to those havingordinary skill in the art from the following detailed descriptiontogether with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

So that those having ordinary skill in the art to which the subjectinvention pertains will more readily understand how to make and use thesubject invention, exemplary embodiments thereof will be described indetail below with reference to the drawings, wherein:

FIG. 1 shows schematically a cross-section of an exemplary hybridpolarizer of the present disclosure;

FIG. 2 shows schematically a cross-section of another exemplary hybridpolarizer of the present disclosure;

FIG. 3 shows schematically a cross-section of yet another exemplaryhybrid polarizer of the present disclosure;

FIG. 4 is a schematic perspective view of a reflective polarizeraccording to the present disclosure;

FIG. 5 is a schematic representation of light incident on a biaxialreflective polarizer at non-zero polar angles (θ) and at azimuth angles(φ) between 0 and 90 degrees;

FIG. 6 is a schematic representation of a display device according toone exemplary embodiment of the present disclosure;

FIG. 7 is a schematic representation of a display device according toanother exemplary embodiment of the present disclosure;

FIG. 8 is a chart showing optical density vs. wavelength of an exemplaryhybrid polarizer of the present disclosure and its components;

FIG. 9 is a chart showing optical density vs. wavelength of anotherexemplary hybrid polarizer of the present disclosure and its components;

FIG. 10 shows a plot of transmissivity of crossed absorbing polarizersas a function of wavelength for angles of incidence from 0 to 75 degreesat the azimuth angle of 45 degrees;

FIG. 11 shows a plot of transmissivity of a D-plate between crossedabsorbing polarizers as a function of wavelength for angles of incidencefrom 0 to 75 degrees at the azimuth angle of 45 degrees;

FIG. 12 shows a plot of transmissivity of a crossed absorbing polarizerand a hybrid polarizer according to the present disclosure as a functionof wavelength for angles of incidence from 0 to 75 degrees at theazimuth angle of 45 degrees;

FIG. 13 shows a plot of transmissivity of a crossed absorbing polarizerand another exemplary hybrid polarizer according to the presentdisclosure as a function of wavelength for angles of incidence from 0 to75 degrees at the azimuth angle of 45 degrees;

FIG. 14 shows a plot of the same characteristics for the same polarizersas in FIG. 12, except that a D-plate is inserted between the polarizers;

FIG. 15 shows a plot of the same characteristics for the same polarizersas in FIG. 13, except that a D-plate is inserted between the polarizers;

FIG. 16 shows a plot of the same characteristics for the same opticalelements as in FIG. 15, with the layer profiles of the reflectivepolarizers reversed such that the majority of the optically thickerlayers faced the analyzer;

FIG. 17 shows a plot of transmissivity of a crossed absorbing polarizerand yet another exemplary hybrid polarizer according to the presentdisclosure as a function of wavelength for angles of incidence from 0 to75 degrees at the azimuth angle of 25 degrees;

FIG. 18 shows a plot of the same characteristics for the same polarizersas in FIG. 17, except that a D-plate is inserted between the polarizers;and

FIG. 19 shows a plot of the same characteristics for the same opticalelements as in FIG. 18, with the layer profiles of the reflectivepolarizers reversed such that the majority of the optically thickerlayers faced the analyzer.

DETAILED DESCRIPTION

The present disclosure is believed to be applicable to hybridpolarizers, which may be suitable for use in display devices. When usedin display devices, such as LCDs, hybrid polarizers according to thepresent disclosure may be used to achieve higher contrast and lowercolor-distortion. The term “hybrid polarizer” refers to a combinationpolarizer, including two reflective polarizers and at least oneabsorbing element in a (reflective polarizer)/(absorbingelement)/(reflective polarizer) stack. The hybrid polarizer can furtherinclude additional optical elements. For example, additional absorbingelements, such as absorbing or dichroic polarizers, may be provided onone or both sides of the hybrid polarizer. Such constructions canprovide high contrast display polarizers for either the back, the front,or both sides of a display panel, such as an LCD panel. For the purposesof the present disclosure, contrast of a polarizer is defined as aphotopically averaged pass state transmission value divided by aphotopically averaged block state transmission value of the polarizer.With an absorbing polarizer attached to a side of the hybrid polarizerthat faces the viewer, the hybrid polarizer can serve the dual functionof providing contrast to a display panel as well as recyclingpolarization for brightness enhancement.

The drawings, which are not necessarily to scale, depict selectedillustrative embodiments and are not intended to limit the scope of thedisclosure. Although examples of construction, dimensions, and materialsare illustrated for the various elements, those skilled in the art willrecognize that many of the examples provided have suitable alternativesthat may be utilized.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the foregoing specification and attached claimsare approximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein.

The recitation of numerical ranges by endpoints includes all numberssubsumed within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3,3.80, 4, and 5) and any range within that range.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” encompass embodiments having pluralreferents, unless the content clearly dictates otherwise. For example,reference to “a film” encompasses embodiments having one, two or morefilms. As used in this specification and the appended claims, the term“or” is generally employed in its sense including “and/or” unless thecontent clearly dictates otherwise.

FIG. 1 shows a hybrid polarizer 100 according to one exemplaryembodiment of the present disclosure, which includes an absorbingelement 140 having a first major surface and a second major surface, afirst reflective polarizer 120 disposed on the first major surface ofthe absorbing element 140, and a second reflective polarizer 160disposed on the second major surface of the absorbing element. Theabsorbing element 140 may be a layer of any suitable material havingnon-zero absorption. The absorbing element may be isotropic orbirefringent. In some exemplary embodiments, the absorbing element maybe an absorbing polarizer having a pass axis and block axis. Lightpolarized along the pass axis of an absorbing polarizer ispreferentially transmitted, while light polarized along the block axisof an absorbing polarizer is preferentially absorbed.

Each of the first and second reflective polarizers 120, 160 has a passaxis and a block axis (first and second, respectively). Light polarizedalong the pass axis of a reflective polarizer is preferentiallytransmitted, while light polarized along the block axis of a reflectivepolarizer is preferentially reflected. Preferably, the first and secondpass axes of the first and second reflective polarizers are aligned asclosely as possible or practicable. The degree of alignment will dependon a particular application. For example, the first and second pass axesmay be aligned to within about +/−10 degrees, about +/−5 degrees, about+/−1 degree, about +/−0.5 degree, or even about +/−0.2 degree. In someexemplary embodiments including an absorbing polarizer, the pass axis ofthe absorbing polarizer may be aligned with the first pass axis, thesecond pass axis, or both, within the constraints described above.

Not intending to be bound by a particular theory, it is believed thatthe need for an absorbing element between the two reflective polarizersis due to the fact that, in the absence of any loss mechanism, half ofthe light that leaks through the first reflective polarizer eventuallyalso leaks through the second reflective polarizer. This occurs due tothe multiple reflections, which are illustrated schematically in FIG. 1.For example, if the first polarizer is 99% reflective and isnon-absorbing, 1% of the block state polarization is transmitted. Uponsumming the infinite number of multiple reflections with the binomialformula, it can be shown that half of this light leaks through thesecond reflective polarizer, if it is also lossless and 99% reflective.Thus, the block state reflectivity of two lossless 99% reflectivepolarizers is not 99.99%, but only 99.5%.

The equation for the overall transmission T of a two-reflector system(representing the reflectivities of light polarized along the block axesof two aligned reflective polarizers) with an absorbing element betweenthe two, derived by summing the infinite series of multiple reflectionsbetween the two reflectors is:

$\begin{matrix}{T = \frac{T_{1}*T_{2}*^{{- \alpha}\; d}}{1 - {R_{2}*R_{2}*^{{- 2}\; \alpha \; d}}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

where T1 and T2 are the transmissivity values of the two reflectors, andR1 and R2 are their corresponding reflectivities. The losses in thereflectors are assumed to be negligible. α and d are the absorptioncoefficient and thickness, respectively, of the absorbing element.Exp(−αd) is the internal transmission of the absorbing element. To afirst approximation, small losses in the reflectors can be included inthis term.

In one exemplary hybrid polarizer according to the present disclosure,the first and second reflective polarizers each have a reflectivity forlight polarized along the first and second block axes not higher thanabout 90%. In such exemplary embodiments, absorption of the absorbinglayer may be about 90% for light polarized in the block direction. Inother exemplary embodiments, one or both reflective polarizers have areflectivity for light polarized along the first and second block axesnot higher than about 95, 96, 98 or 99%.

Another exemplary embodiment of a hybrid polarizer 200 constructedaccording to the present disclosure is shown in FIG. 2. The hybridpolarizer 200 includes an absorbing element 240 having a first majorsurface and a second major surface, a first reflective polarizer 220disposed on the first major surface of the absorbing element 240, and asecond reflective polarizer 260 disposed on the second major surface ofthe absorbing element. In some exemplary embodiments, the absorbingelement 240 may be an absorbing polarizer having a pass axis and blockaxis. In other exemplary embodiments, the absorbing element 240 may beisotropic or nearly isotropic.

The hybrid polarizer 200 further includes an anti-reflective oranti-glare layer 230 disposed, for example, on the viewer side 200 a ofthe hybrid polarizer 200 (i.e., the side of the hybrid polarizer that isintended to face a viewer when the hybrid polarizer is installed into adisplay device). The anti-reflective layer 230 may be another absorbingelement, such as an absorbing polarizer having a pass axis and a blockaxis. In one exemplary embodiment, the anti-glare layer 230 is arelatively low contrast absorbing polarizer layer used to eliminate theglare from the viewer side 200 a of the hybrid polarizer 200. Theabsorbing anti-glare layer also enables a contrast improvement. In suchexemplary embodiments, the absorbing element disposed between the tworeflective polarizers can have a lower contrast ratio than theanti-glare layer, to the point of being isotropic. The anti-glare oranti-reflective element 230 can be a layer including dyes coextrudedwith one or more of the other elements of the hybrid polarizer 200, orthe anti-reflective element 230 may be coated or laminated onto anotherelement of the hybrid polarizer 200.

Each of the first and second reflective polarizers 220, 260 has a passaxis and a block axis (first and second, respectively). Preferably, thefirst and second pass axes of the first and second reflective polarizersare aligned as closely as possible or practicable, as described above.In the exemplary embodiments where one or both of the elements 240 and230 are absorbing polarizers, one or both of their pass axes may bealigned with the first pass axis, the second pass axis, or both withinthe constraints provided above.

Yet another exemplary embodiment of a hybrid polarizer 300 constructedaccording to the present disclosure is shown in FIG. 3. The hybridpolarizer 300 includes an absorbing element 340 having a first majorsurface and a second major surface, a first reflective polarizer 320disposed on the first major surface of the absorbing element 340, and asecond reflective polarizer 360 disposed on the second major surface ofthe absorbing element. The hybrid polarizer 300 may or may not furtherinclude an anti-reflective or anti-glare element 330 disposed on theviewer side 300 a of the hybrid polarizer 300. The hybrid polarizer 300includes a rear absorbing element 350, which may be an absorbingpolarizer. The rear absorbing element 350 is disposed on the back side300 b of the hybrid polarizer 300, which is opposite to the viewer side300 a. In some exemplary embodiments, one, two or all absorbing elements330, 340 and 350 may be absorbing polarizers having a pass axis andblock axis. One or both of the anti-glare element 330 and the rearabsorbing element 350 may sometimes require a different absorber type orconcentration than the absorbing element 340 between the reflectingpolarizers.

The exemplary hybrid polarizer 300 may be used as the front (viewerside) display polarizer of a display device. The rear absorbing element350 would minimize multiple reflections from elements in the displaydevice and the hybrid polarizer. Each of the first and second reflectivepolarizers 320, 360 has a pass axis and a block axis (first and second,respectively). Preferably, the first and second pass axes of the firstand second reflective polarizers are aligned as closely as possible orpracticable. In the exemplary embodiments where one, two or all of theabsorbing elements 330, 340 and 350 are absorbing polarizers, one, twoor all of their pass axes may be aligned with the first pass axis, thesecond pass axis, or both within the constraints stated above.

Reflective Polarizer

One or both reflective polarizers used in exemplary hybrid polarizersaccording to the present disclosure may be birefringent reflectivepolarizers. FIG. 4 illustrates one exemplary embodiment of a reflectivepolarizer according to the present disclosure, which is a multilayeroptical film 111 that includes a first layer of a first material 113disposed (e.g., by coextrusion) on a second layer of a second material115. The depicted optical film 111 may be described with reference tothree mutually orthogonal axes x, y and z. Two orthogonal axes x and yare in the plane of the film 111 (in-plane, or x and y axes) and a thirdaxis (z-axis) extends in the direction of the film thickness. One orboth of the first and second materials may be birefringent.

While only two layers are illustrated in FIG. 4 and generally describedherein, typical embodiments of the present disclosure include two ormore of the first layers interleaved with two or more of the secondlayers. The total number of layers may be hundreds or thousands or more.In some exemplary embodiments, adjacent first and second layers may bereferred to as an optical repeating unit. Reflective polarizers suitablefor use in exemplary embodiments of the present disclosure are describedin, for example, U.S. Pat. Nos. 5,882,774, 6,498,683, 5,808,794, whichare incorporated herein by reference.

The optical film 111 may include additional layers. The additionallayers may be optical, e.g., performing an additional optical function,or non-optical, e.g., selected for their mechanical or chemicalproperties. As discussed in U.S. Pat. No. 6,179,948, incorporated hereinby reference, these additional layers may be orientable under theprocess conditions described herein, and may contribute to the overalloptical and/or mechanical properties of the film, but for the purposesof clarity and simplicity these layers will not be further discussed inthis application. For the purposes of the present disclosure, it ispreferred that thick biaxially birefringent outer layers are notdisposed on the side of the polarizer that faces a display panel. Ifthick outer layers are needed on the side of the polarizer that isintended to face the display once installed, such layers should beremovable or they should be made of isotropic or only weakly biaxiallybirefringent materials.

In a birefringent reflective polarizer, the refractive indices of thefirst layers 113 (n_(1x), n_(1y), n_(1z)) and those of the second layers115 (n_(2x), n_(2y), n_(2z)) are substantially matched along onein-plane axis (y-axis) and are substantially mismatched along anotherin-plane axis (x-axis). The matched direction (y) forms a transmission(pass) axis or state of the polarizer, such that light polarized alongthat direction is preferentially transmitted, and the mismatcheddirection (x) forms a reflection (block) axis or state of the polarizer,such that light polarized along that direction is preferentiallyreflected. Generally, the larger the mismatch in refractive indicesalong the reflection direction and the closer the match in thetransmission direction, the better the performance of the polarizer.

To function well for wide angle viewing of a display device, a displaypolarizer should maintain high block state contrast for all angles ofincidence and also maintain high pass transmission over all angles ofincidence. As it has been shown in the commonly owned U.S. Pat. No.5,882,774, pass state transmission increases when the refractive indicesof the alternating first and second layers 113 and 115 are matched forlight polarized along the z-axis and for light polarized along they-axis. The z-index matching also ensures that the block statetransmission does not degrade at high angles of incidence.

Preferably, at least one reflective polarizer in a hybrid polarizeraccording to the present disclosure is nearly uniaxial. For the purposesof the present disclosure, “nearly uniaxial” is defined asΔn_(yz)=|n_(y)−n_(z)| being less than or equal to about 0.05 at 633 nmfor a particular birefringent polarizer material. In some exemplaryembodiments of nearly uniaxial birefringent reflective polarizers,Δn_(yz) may be about 0.03 or less, about 0.02 or less, about 0.01 orless, or about 0.005 or less. More preferably, the first and secondreflective polarizers of the hybrid polarizers constructed according tothe present disclosure are both nearly uniaxial. Even more preferably,all components of the hybrid polarizer are either nearly uniaxial orsubstantially isotropic.

In other exemplary embodiments, at least one reflective polarizer in ahybrid polarizer may be biaxial, that is, having Δn_(yz) of more thanabout 0.05 for a particular birefringent polarizer material. In otherexemplary embodiments, Δn_(yz) can be at least 0.08 or another suitablevalue depending on the application. In some exemplary embodiments,Δn_(yz) can be no more than about 0.1. All values of refractive indicesand refractive index differences are reported for 633 nm.

Although biaxial reflective polarizers can have low reflectivity forlight polarized parallel to the pass axis (y) for any angle ofincidence, when light is incident onto the reflective polarizer atnon-zero polar angles (θ) and at azimuth angles (φ) between 0 and 90degrees (see FIG. 5), the magnitude of the reflectivity can oscillatedramatically as a function of both angle of incidence and wavelength ofthe incident light. This is believed to be at least in part due tounequal conversion of s-polarization to p-polarization andp-polarization to s-polarization as a function of wavelength as lighttraverses the biaxial medium. This phenomenon both increases appearanceof undesirable color of the display device and lowers the contrast ofthe reflective polarizer when it is crossed with another reflectivepolarizer or with an absorbing polarizer. When a polarizer used in adisplay is crossed with another polarizer, a uniform extinction vs.wavelength spectrum is desired for all azimuths (φ) from 0 to 360degrees, i.e., not just for the planes of incidence parallel to theblock and pass axes but for all azimuthal angles (φ) between these axes.

The present disclosure provides a construction for a hybrid polarizerincluding a biaxial reflective polarizer that yields improved pass statetransmission and improved contrast for all azimuthal angles at non-zeropolar angles of incidence. This construction includes, for example, abiaxial reflective polarizer constructed of alternating low and highindex layers, e.g., first and second layers 113 and 115, with theoptical thickness of the repeat unit d₁*n_(x1)+d₂*n_(x2) at normalincidence being of ½λ thickness and wherein the repeat units, as well asthe constituent layers, are arranged such that a majority of the layershaving a smaller optical thickness d*n (referred to as “blue” layers)are disposed closer to a display panel than the layers having a largeroptical thickness (referred to as “red” layers). When such a polarizingfilm is crossed with itself or with an absorbing polarizer, theextinction is much better at more angles around the azimuth, if the bluelayers are closest to the other polarizer.

Preferably, the profile of the optical thicknesses of the layers in thethickness direction of the reflective polarizer is a monotonic function,or at least, a majority of the layers characterized by a varying opticalthickness are disposed such that their optical thicknesses decreasemonotonically in the direction toward the display panel. However, insome exemplary embodiments, the function characterizing the profile ofoptical thicknesses of the layers in a biaxial reflective polarizer mayhave local minima and maxima. These minima and maxima can bedisregarded, so long as the majority of the layers having a smalleroptical thickness are disposed closer to the display panel than thelayers having a larger optical thickness, as described in commonly ownedU.S. Patent Application 3M Docket No. 62631US002, filed on even dateherewith, the disclosure of which is incorporated by reference herein.

Other exemplary reflective polarizers suitable for use in hybridpolarizers according to the present disclosure are also described inU.S. Pat. No. 6,697,195, hereby incorporated by reference herein.

Absorbing Elements

An absorbing element can be stacked with reflective polarizers,laminated to one or more reflective polarizers, co-extruded with one ormore reflective polarizers or coated onto and oriented with one or morereflective polarizers. In some exemplary embodiments, an entire(reflective polarizer)/(absorbing element)/(reflective polarizer)combination may be coextruded as a single film, or parts of it can beseparately extruded and laminated, or first oriented and then laminated.

Generally, any optically absorbing structure can be used as theabsorbing element depending, at least in part, on the desiredwavelengths of absorption and transmission. One example of an absorbingelement includes a layer of light absorbing material, such as, forexample, dye, pigment, or ink disposed in a supporting matrix or on asupporting substrate.

Suitable absorbing elements include glass filters, such as thoseobtainable from Schott Glass Technologies, Inc., Duryea, Pa., includingthe KG series of heat control filters which absorb strongly in the nearinfrared but are relatively transparent in the visible. GentexCorporation (Carbondale, Pa.) makes plastic optical filters under thetrade name Filtron™. In addition, polycarbonate or acrylic sheets loadedwith dyes absorb at various wavelengths across the visible and IR.Suitable IR and visible absorbing dyes include dyes with good thermalstability that can be injection molded with, for example, polycarbonate.Other suitable dyes have broad solubility and are recommended forsolution coating. Alternative absorbing materials include pigments suchas carbon black and iron oxides, such as iron oxide-loaded glass.

The selection of the light absorbing material can be made based onfactors, such as, for example, the absorbance spectrum of the lightabsorbing material, cost, processibility, stability, and compatibilitywith other elements in the optical filter. A light absorbing materialmay be selected with an average absorptance of at least about 5%, 10%,20%, 30%, or 50% over the wavelength range that is to bereflected/absorbed. The light absorbing material may have a relativelylow average absorptance (e.g., no more than 40%, 20%, 10%, 5%, or 1%)over the wavelength range where transmission is desired. It will beappreciated, however, that many light absorbing materials suitable forbroadband absorptive elements have substantial absorbance over arelatively wide range of wavelengths or a relatively constantabsorptance value over portions of both the transmission and reflectionwavelength ranges. The use of the combination of an absorptive elementbetween two reflective polarizers can allow the use of lower loadings oflight absorbing material than if the absorptive element was used aloneor with a single reflective element.

Many other types of lossy elements can be used, including, for example,lossy elements that employ scattering or a combination of scattering andabsorption. For example, depending on particle size, pigments or otherparticles used in the optical filters can scatter light rays. Althoughthis may introduce additional haze, a scattering loss is typicallyequivalent to an absorptive loss. Generally, scattering is only slowlywavelength dependent and is typically stronger for shorter wavelengths.Scattering can be polarization dependent based on the shape of thescattering particles.

As mentioned above, absorbing polarizers are also suitable for use inexemplary embodiments of the present disclosure. One useful polarizingabsorptive element is an oriented, dye-containing, polyvinyl alcohol(PVA) film. Examples of such films and their use as polarizingabsorptive elements are described, for example, in U.S. Pat. Nos.4,895,769, and 4,659,523 and PCT Publication No. WO 95/17691, all ofwhich are incorporated herein by reference. To function as an absorbingpolarizer, the polyvinyl alcohol film is typically stretched to orientthe film. When stained with a polarizing dye or pigment, the orientationof the film determines the optical properties (e.g., the axis ofextinction) of the film. Preferably, the absorbing element is such thatabsorption of light polarized along the block axis does not decrease(and sometimes increases) with increased angle of incidence. One exampleof such absorbing elements are absorbing polarizers includingsupra-molecular lyotropic liquid-crystalline material, as described inLazarev et al. article, entitled “Low-leakage off-angle in E-polarizers,Journal of the SID 9/2, pp. 101-105 (2001), incorporated by referenceherein.

Absorbing polarizers used in exemplary embodiments of the presentdisclosure have a contrast ratio of less than 1000:1, thus making thecontribution of the reflective polarizers more important. In someexemplary embodiments, the contrast ratios of absorbing polarizers maybe about 500:1 or less, about 100:1 or less, about 10:1 or less, orabout 5:1 or less. In some exemplary embodiments, the absorbingpolarizer may be characterized by a contrast ratio of about 5:1 to about100:1.

Where the absorbing polarizer of the reflective/absorbing/reflectivepolarizer combination has a contrast ratio of up to about 10:1, at leastone of the reflective polarizers preferably has a contrast of at leastabout 100:1. In other exemplary embodiments, one or both of the biaxialreflective polarizers may be characterized by a contrast ratio of atleast about 50:1, at least about 100:1 or at least about 200:1. Thereflective/absorbing/reflective polarizer combination according to thepresent disclosure may have a total contrast ratio of about 500:1 ormore or about 1000:1 or more. In some exemplary embodiments, thecontrast ratio of the reflective/absorbing/reflective polarizercombination according to the present disclosure may be as high as about10,000:1.

Display Devices Including Hybrid Polarizers

Hybrid polarizers described above are believed to be useful in severaltypes of display devices, such as LCDs. They can function as areplacement for high performance absorbing polarizers on either theviewer side or the rear side of a display panel. When the constructionshown in FIG. 2 is used on the rear side of a display panel, the hybridpolarizer can function as both a display polarizer and a backlightpolarization recycling film. The construction of FIG. 2 can also be usedas a viewer side polarizer of a display panel to recycle unused lightfrom off-state pixels to the backlight in order to provide for increasedbrightness to the on-state pixels.

With a hybrid polarizer construction, the reflective polarizer of thethree polarizer stack that is adjacent the display should be arranged tohave the blue to red layer construction just described, with the bluelayers closest to the display panel. The construction of the otherreflective polarizer, on the opposite side of the absorbing layer, isnot as important. An exemplary display device 400 having a hybridpolarizer 440 disposed on the rear side of a display panel 420 isillustrated in FIG. 6, where the viewer is on the left. As shown, thehybrid polarizer 440 includes an absorbing element 444 having a firstmajor surface and a second major surface, a first birefringentreflective polarizer 446 disposed on the first major surface of theabsorbing element and a second birefringent reflective polarizer 442disposed on the second major surface of the absorbing element, and ananti-reflective element 443 disposed on the viewer side of the hybridpolarizer 440. Those of ordinary skill in the art will readilyappreciate that the hybrid polarizer 440 may have any of theconfigurations shown in FIGS. 1-3.

In this exemplary embodiment, the second reflective polarizer 442 isdisposed closer to the display panel than the first reflective polarizer446, but in other exemplary embodiments, the order may be reversed.Preferably, the reflective polarizer that is disposed closer to thedisplay panel includes a plurality of layers characterized by a varyingoptical thickness, in which a majority of the layers having a smalleroptical thickness are disposed closer to the display panel than thelayers having a larger optical thickness. The display device 400 furtherincludes a compensation film 470 and an additional display polarizer410.

Another exemplary display device 500 having a hybrid polarizer 540disposed on the viewer side of a display panel 520 is illustrated inFIG. 7, where the viewer is on the left. As shown, the hybrid polarizer540 includes an absorbing element 544 having a first major surface and asecond major surface, a first reflective polarizer 546 disposed on thefirst major surface of the absorbing element and a second reflectivepolarizer 542 disposed on the second major surface of the absorbingelement, and an anti-reflective element 543 disposed on the viewer sideof the hybrid polarizer 540. Those of ordinary skill in the art willreadily appreciate that the hybrid polarizer 540 may have any of theconfigurations shown in FIGS. 1-3.

In this exemplary embodiment, the second reflective polarizer 542 isdisposed closer to the display panel than the first reflective polarizer546, but in other exemplary embodiments, the order may be reversed.Preferably, the reflective polarizer that is disposed closer to thedisplay panel includes a plurality of layers characterized by a varyingoptical thickness, in which a majority of the layers having a smalleroptical thickness are disposed closer to the display panel than thelayers having a larger optical thickness. The display device 500 furtherincludes a compensation film 570 and an additional display polarizer510. The additional polarizer 410 or 510 may also be a hybrid polarizeraccording to the present disclosure.

Typically the compensation film 470, 570 may be disposed between thedisplay panel 420, 520 and the hybrid polarizer 440, 540, between thedisplay panel 420, 520 and the additional polarizer 410, 510, or both.One example of a suitable compensation film is a biaxial birefringentfilm. One type of a biaxial birefringent film is termed a “D-plate”, anexample of which is the NRZ™ film available from Nitto Denko Corporationof Osaka, Japan. Such a film has an out of plane retardation R_(th) thatis approximately 0, where R_(th) is given byR_(th)=[(n_(x)+n_(y))/2−n_(z)]*thickness. That is, the z-index of theD-plate is approximately equal to the average of the x and y indices ofrefraction of the film. A typical D-plate compensation film also has anin-plane retardation R₀=(n_(x)−n_(y))*thickness that is approximatelyequal to ½λ, where λ is in the wavelength range of interest. Thecompensation film(s) also may be designed to correct for angle-dependentretardation of the LC material. To this end, additional retardance, inan amount equal but opposite in sign to the LC material, is added to thecompensation layer(s) to correct for the retardance of the LC materialin a complete LC display panel.

Exemplary display devices according to the present disclosure mayinclude a backlight as known to those of skill in the art. In theexemplary embodiments including a backlight, a hybrid polarizer may bedisposed between the backlight and the display panel. The configurationof the backlight is not limited to any specific construction. Anysuitable structure capable of providing light to the display panel maybe used. Suitable examples of backlights include, without limitation,edge-lit backlights including one or more light sources opticallycoupled to one or more edges of one or more lightguides, and direct-litbacklights including one or more light sources disposed such that thedisplay panel is disposed between the one or more light sources and aviewer, that is, directly behind the display panel in the field of viewof a viewer of the display. In the exemplary embodiments including aback reflector and no backlight, the hybrid polarizer may be disposedbetween the reflector and the display panel.

Other optical elements and films may be included into display devicesaccording to the present disclosure as would be known to those ofordinary skill in the art. Exemplary suitable additional opticalelements include, without limitation, structured surface films. Examplesof structured surface films include films having a plurality of linerprismatic surface structures, a plurality of lenticular surfacestructures, a matrix or a random array of surface structures, andothers. Another type of an optical film suitable for use in displays ofthe present disclosure are optical films including a layer includingbeads dispersed in a binder. Similarly, diffuser films used to increasethe uniformity of illumination could also be disposed at variouslocations such as between the backlight and the biaxial reflectingpolarizer film. Such films may be disposed between the backlight and thebiaxial reflective polarizer or at another suitable location.

Exemplary hybrid polarizers of the present disclosure may be capable ofproviding contrast of 10,000:1, which would be exceptionally valuable inprojection displays, either as a polarizing beamsplitter or aprepolarizer. For such applications, the preferred construction would bethe one illustrated in FIG. 1.

Bandwidth

Although most LCD displays are broadband, i.e., they control thetransmission of most wavelengths of visible light, hybrid polarizers ofthe present disclosure can be either broadband or narrow band. Forexample, hybrid polarizers according to the present disclosure and canoperate in the UV, visible, or infrared portions of the spectrum, or inany combination of the three. The bandwidth of the absorbing elementsand the anti-glare elements typically need only absorb over the range ofwavelengths to which the reflective polarizers are tuned, but may bebroader or narrower than those ranges, depending on the application. Thetwo reflective polarizers would typically operate over the same range ofwavelengths, but may be wavelength shifted with respect to each other asdesired. For example, the two reflective polarizers may be wavelengthshifted so that identical spectral leaks do not align.

EXAMPLES

The examples below explore some of the performance characteristics of ahybrid polarizer including a dichroic dye absorbing polarizer disposedbetween two multilayer reflective polarizers.

Example 1

A construction shown in FIG. 1 was produced, using Advanced PolarizerFilm (APF), available from 3M Company, as reflective polarizers. Asuitable APF film has been described, for example in the Invited Paper45.1, authored by Denker et al., entitled “Advanced Polarizer Film forImproved Performance of Liquid Crystal Displays,” presented at Societyfor Information Displays (SID)International Conference in San Francisco,Calif., Jun. 4-9, 2006.

Each APF reflective polarizer was made of 275 alternating layers of90/10 coPEN, a polymer composed of 90% polyethylene naphthalate (PEN)and 10% polyethylene therephthalate (PET), and a low index isotropiclayer, which was made with a blend of polycarbonate and copolyesterssuch that the index is about 1.57 and remains substantially isotropicupon uniaxial orientation of the coPEN. The absorbing layer was madewith a magenta dichroic dye mixed with PVA, which was then coated onto aPET cast web and then uniaxially oriented in a batch stretcher. Thedichroic coated PET layer was then laminated between the two APF filmswith the block axes of all three parallel to one another.

Using Equation 1, the predicted optical density (OD, here defined as−Log(T)) was calculated and is shown in FIG. 8 as the Theoretical OD.The OD of the laminate, measured in a Perkin Elmer λ-19spectrophotometer against an integrating sphere, is shown in the chartas the Measured OD. The samples were measured with pre-polarized lightfrom a Glan-Thompson crystal polarizer. The transmission of each of thethree polarizers in the hybrid stack was also measured. The chart inFIG. 8 shows the measured transmission spectrum of the two reflectivepolarizers, referred to as APF1 and APF2, and of the absorbing layer,referred to as Dichroic dye.

Several things can be noted from this chart. In the region of longerwavelengths, the dye has essentially zero absorption, so the OD of thecombined polarizers is only a little more than that of one reflectivepolarizer, as predicted by equation 1. The measured OD is slightlyhigher than theoretical, which implies that there is a small amount ofloss in the laminate, which may be due to either scattering orabsorption, or both. At shorter wavelengths, where the dye is absorbingand the reflective polarizers have higher OD, the theoretical OD isquite high. However, the measured OD is lower, although a peak OD ofabout 3.5 was obtained. The sample was then re-measured with a cleanuppolarizer oriented parallel to the pass state direction. The measured ODthen was equal to or above the theoretical values. Since no measurablepass state light was incident on the laminate from the crystalpolarizer, this implies that some of the block state light was convertedto pass state light. This is indicative of scattering in the films,which depolarizes some of the incident light.

Example 2

For this sample, two slightly less reflective APF films were laminatedto a broadband absorbing polarizer, which had between 90% and 95%absorption of block state light. The absorbing polarizer was made bycoating a mixture of red, green and blue lyotropic dyes onto apolymethyl methacrylate (PMMA) film. The water soluble lyotropic dyeswere oriented by the shearing action in coating process and thenimmediately dried. The OD of the absorbing polarizer is shown in FIG. 9,labeled as “dichroic dye.” As in the previous example, the OD of thelaminate was calculated using equation 1 and is plotted as thetheoretical OD. The OD of the laminate was measured with a Perkin-ElmerX-950 spectrophotometer with light, which was pre-polarized by aGlan-Thompson polarizer. Spectra were obtained with and without acleanup polarizer as in the previous example. The average OD is about3.7 without the clean-up polarizer and is about 5.0 with the clean-uppolarizer. The latter approximately matches the theoretical value. Thedifference between the two can again be ascribed to scattering anddepolarization of the polarized measurement beam. In order to minimizescattering, the material should be made with as clean a resin aspossible, and scattering due to polymer crystallites should also beminimized. The latter can be effected with the use of birefringentcopolymers as copolymers tend to have smaller crystal sizes thanhomopolymers. As in known in the art, orientation conditions can alsoaffect the crystallite size in the polymer.

Comparative Example 3

The data in FIGS. 8 and 9 were obtained at near-normal incidence.However, an LCD comprising crossed polarizers should have a highcontrast at all angles of incidence and azimuth. In this ComparativeExample, the transmission of crossed absorbing polarizers was calculatedas a function of wavelength for angles of incidence from 0 to 75degrees. Thick protective layers made from a birefringent material suchas cellulose triacetate (TAC) were not included in the modeled polarizerconstruction. The plane of incidence was at the azimuth angle of 45degrees. These spectra are shown in FIG. 10. Note that the averagevisible light transmission at 60 degrees is about 1.5%, which is thevalue typically observed with crossed absorbing polarizers.

The insertion of a D-plate between the two absorbing polarizersdramatically reduces the transmission of light at the higher angles ofincidence. This result is illustrated in FIG. 11, which has 10× expandedscale as compared to FIG. 10. The photopic weighted visible lighttransmissivity at 60 degrees is now only about 0.0005, which is muchless than the value of 0.015 illustrated in FIG. 10 for the case with nocompensation. Approximately the same transmission, or lower, is obtainedfor other azimuthal angles.

The D-plates used in this and the following modeled examples wereassumed to have an in-plane retardance of ½λ at 530 nm and were assumedto be made of a material with the nominal indices and dispersion valuesof polycarbonate. The thickness of the plate was assumed to be 10microns and the in-plane indices were assumed to be n_(x)=1.592 andn_(y)=1.566 at 633 nm. Given these in-plane indices, the ideal D-platehas a value of n_(z)=1.579.

Example 4

Transmissivity of a hybrid polarizer having the construction of FIG. 6,using APF for the two reflective polarizers, and crossed with one of theabsorbing polarizers of the previous comparative example, was modeled atthe azimuth angle of 45 degrees in the same manner as described above.The high refractive index layers were assumed to have a y-z indexmismatch of about 0.015. Both the absorbing layer between the APFstructures, as well as the anti-reflective (AR) top layer, are assumedto have the same real indices as the high refractive index layers of theAPF but have the same imaginary (absorbing) indices as the absorbingpolarizers. The absorbing layer was assumed to have an absorption ofabout 50% of block state polarization and the anti-reflective layer wasassumed to have an absorption of about 80% of light with the block statepolarization.

The spectra without a D-plate between the crossed polarizers are shownin FIG. 12. Note that the spectra are similar to those in FIG. 10 forabsorbing polarizers, except for some ripple in the spectra at highangles of incidence. This ripple was found to be due to the biaxialnature of the anti-reflective layer. If this anti-reflective layer isreplaced with a nearly uniaxial layer such as a PVA iodine polarizerlayer with the same absorptivity, the spectra of FIG. 13 are obtained.Note that these are also slightly higher than the values in FIG. 10 forthe crossed absorbing polarizers. However, if all layers of the hybridpolarizer are assumed to be true uniaxial, approximately the same resultis obtained as in FIG. 10.

When a D-plate is inserted between the polarizers, however, thetransmission, shown in FIGS. 14 and 15 for the two cases just described,drops to about the values observed for the crossed absorbing polarizers.Note that for the case with a uniaxial AR layer, shown in FIG. 15, thespectra are almost identical to those of crossed absorbing polarizers.As in the case above for absorbing polarizers, approximately the same orlower transmission is calculated for more azimuthal angles. This showsthat a hybrid polarizer constructed with a very low contrast AR layercan be used in a display to produce high contrast over a wide range ofviewing angles similar to high performance absorbing polarizers.

Example 5

Example 4 illustrates that the hybrid polarizer does not have to beconstructed with purely uniaxial material layers. The present Example 5shows that it is desirable to have the thinnest layers of such aconstruction facing the analyzer. When the layer profiles of thereflective polarizers in Example 4 were reversed such that the majorityof the optically thicker layers faced the analyzer, the spectra of FIG.16 were obtained. Although the average transmission is about the same, ahigher frequency ripple is introduced into the spectra, which can causea higher incidence of perceived color when the display is illuminatedwith narrow band RGB light sources. Thus the construction of Example 4is preferred.

Example 6

Example 4, with the not perfectly uniaxial coPEN layers in the APF(Δn_(yz)=0.015) raises the question of how much birefringence of thelayers can depart from uniaxial and still provide a high contrast, i.e.compensatable, display polarizer. To test this limit, a hybrid polarizerwas modeled using layers having the indices of standard materials usedfor the reflective polarizer referred to as Vikuiti™ Dual BrightnessEnhancement Film (DBEF), available from 3M Company, for whichΔn_(yz)≈0.08 in the high index layers. (n_(x)≈1.8, n_(y)≈1.62,n_(z)≈1.54.) Since n_(y)=1.62 the low index material also hasn_(x)≈1.62, giving Δn_(x)≈0.18 between the high and low index layers. Toobtain a high contrast construction, stacks of 550 layers in eachreflective polarizer were modeled as a laminate with biaxial absorbingand AR layers. The latter two layers were assumed to be absorbing layerswith real index values of 1.8, 1.62, and 1.54. Thicknesses were 10microns and 15 microns respectively, as in Examples 4 and 5. Theconstruction of FIG. 6 was modeled for an azimuth of 25 degrees, withblue layers facing the analyzer, resulting in the spectra of FIG. 17.The irregular spectra and high transmission at 60 and 75 degrees resultin rather low contrast at those angles.

The transmission of the spectra in FIG. 17 can be substantially reducedif the dichroic AR and absorbing layers are uniaxial. This may bepossible if those layers are coated, as for a PVA layer, but then thecost advantage of co-extrusion is not realizable. An alternative is touse modified or additional compensation films. For example if theD-plate (n_(x)=1.5917, n_(y)=1.5655, n_(z)=1.5794) is replaced with afilm with indices of n_(x)=1.5917, n_(y)=1.5655, n_(z)=1.5731, thespectra of FIG. 18 are obtained. This is only a slight reduction in thez-index of the D-plate, which is rather surprising given that thebiaxial material in this hybrid polarizer construction has a lowz-index. The spectra of this example have transmissions almost as low asfor the uniaxial case (see FIG. 18).

For comparison, the same biaxial hybrid polarizer layers were modeledwith the red layers facing the analyzer. Using the same modifiedD-plate, the spectra of FIG. 19 were obtained.

Advantages of exemplary embodiments of the present disclosure includethe possibility of using only one high extinction reflective polarizerin the hybrid polarizer. This can result in reduction of visibility ofoptical defects in the finished display polarizer. An optical defect isany film defect that causes a noticeable leakage of light when theoptical axes of two polarizers are crossed and viewed above a backlight.Such defects can be due to a local disruption of the optical layers by aparticle, or can be due to layer profile errors on a larger scale, suchas along a line or in an extended area, e.g., caused by disruption ofthe laminar polymer flow during extrusion. Other spectral leaks may beinherent in the film due to the design of the extrusion hardware thatgenerates the layers. When two reflective polarizers are utilized, theprobability of aligned defects is small, the end result being that lightleaking through a defect in one polarizer is at least partially blockedby the second polarizer.

Another advantage of exemplary embodiments of the present disclosure isthe use of reflective polarizers with lower layer counts with a layer oflow contrast absorbing dyes. In the configuration shown in FIGS. 1-3,each of the reflective polarizers needs only a modest level ofreflectivity, for example, on the order of or less than 99%, which is aperformance level achievable with only about 200 to 300 layers withcurrently available resin materials. With relatively low polarizingdemands on the absorbing layer, a wide range of polarizing dyes can beused in this construction, and these dyes can be chosen to maximize thetransmission of the pass state polarization.

Although the polarizers and devices of the present disclosure have beendescribed with reference to specific exemplary embodiments, those ofordinary skill in the art will readily appreciate that changes andmodifications may be made thereto without departing from the spirit andscope of the present disclosure. In particular, although a specificdisplay element has not been specified in any of the preceeding examplesit should be appreciated that such examples approximately describedevices using a liquid crystal display based on in-plane switching(IPS). As mentioned previously, other types of displays can be employedwhich may be compensated with additional or alternative elements otherthan the D-plate.

1. A hybrid polarizer, comprising: an absorbing element having a firstmajor surface and a second major surface, wherein the absorbing elementcomprises a light absorbing material having an average absorptance of nomore than 40% over the desired wavelength range; a first nearly uniaxialbirefringent reflective polarizer disposed on the first major surface ofthe absorbing element, the first nearly uniaxial birefringent reflectivepolarizer having a first pass axis and a first block axis; and a secondbirefringent reflective polarizer disposed on the second major surfaceof the absorbing element, the second reflective polarizer having asecond pass axis and a second block axis.
 2. The polarizer of claim 1,wherein the first and second reflective polarizers each have areflectivity for light polarized along the first and second block axesthat is from about 90% to about 99%.
 3. The polarizer of claim 1,wherein the first and second pass axes are aligned to within about +/−1degrees.
 4. The polarizer of claim 1, wherein at least one of the firstand second reflective polarizers comprises a plurality of layerscharacterized by a varying optical thickness.
 5. The polarizer of claim1, wherein the first reflective polarizer has a refractive indexdifference for light polarized along the first pass axis and lightpolarized along a thickness direction of the first reflective polarizerthat is about 0.02 or less at 633 nm.
 6. The polarizer of claim 1,wherein the second reflective polarizer is nearly uniaxial.
 7. Thepolarizer of claim 1, further comprising an additional absorbing elementdisposed on the first or second reflective polarizer.
 8. The polarizerof claim 1, wherein a transmission spectrum of the first reflectivepolarizer is not the same as the transmission spectrum of the secondreflective polarizer.